1. Field of the Invention
This invention relates to a circuit for starting a high intensity, gaseous discharge lamp and particularly to such a circuit that provides suitable starting pulses until such a lamp is lit and automatically removes the pulses after the arc in the lamp reaches a sustaining condition.
2. Description of the Prior Art
Some high intensity, gaseous discharge lamps require the application of suitable high voltage pulses in order to start the lamp. Typical of such lamps is the increasingly popular high pressure sodium (HPS) lamp that requires appropriate starting pulses on the order of several kilovolts. The purpose of the pulses is to initiate ionization of the gas inside the arc tube and thereby permit current to flow from the ballast. The pulses continue for a short time after initial striking of the lamp until the lamp warms up and normal operation occurs. At that time, the starting pulses can be, and usually are, removed, the operation being maintained at that point by a current flow from the ballast.
The parameters of the starting pulse are defined by the American National Standards Institute (ANSI) and relate to pulse amplitude, pulse width, pulse repetition rate and the position of the pulse with respect to the peak of the ballast ac voltage output waveform. For example, the pulse should occur in time sequence in a specified proximity with the peak of the ballast voltage waveform.
The circuits for supplying starting pulses to high pressure sodium lamps are referred to in the lighting industry as "HPS lamp starting aids". Because of the necessity of meeting the ANSI starting pulse parameters, most circuits comprise a controlled pulse discharge circuit governed by a timing circuit. Typically, such a prior art circuit includes a capacitor which is allowed to charge, and then discharge, through a portion of the windings of the ballast. By transformer action of the ballast, the pulse voltage applied thereto is stepped up to produce the desired pulse output to the lamp.
For describing more in detail a typical prior art HPS lamp starting aid, reference may be had to FIGS. 1 and 2. FIG. 1 shows typical connections for a starting aid 2 with respect to a ballast 4, lamp 6, power distribution line 8 and, in most cases, a power factor capacitor 9. FIG. 2 shows the typical components of a prior art starting aid, which is connected in FIG. 1 as aid 2. When the lamp is off, it appears as an open circuit to the ballast and to aid 2. Capacitor 10 charges up through power resistor 12 connected to the "line" connection. At the same time, capacitor 14 is charged through resistor 16. A resistor 18 is connected in parallel with capacitor 14 and, hence, controls the charging rate of capacitor 14 and also allows capacitor 14 to more completely discharge at the time of discharge.
When the voltage on capacitor 14 becomes sufficiently large so as to exceed the breakover voltage of diac or silcon bilateral switch 20, then this device conducts and supplies gate voltage to thyristor 22. Thyristor 22 has typically been an SCR in the prior art. The conduction of SCR 22 discharges capacitor 10 therethrough, thereby applying a pulse to the "tap" connection of the ballast and, hence, through a few windings thereof. Resistor 15 is connected between the gate of SCR 22 and the "tap" connection of the transformer. It is the gate return resistor to SCR 22 and provides leakage current bypass and noise immunity. Via transformer action of the ballast, it may be seen by referring to FIGS. 1 and 2 together that the appropriate pulse is applied to lamp 6, lamp 6 also being connected to the "lamp" connection of the ballast.
Assuming that thyristor 22 is a typical SCR unidirectional device, the timing/discharge cycle or sequence just described occurs 60 times per second for applied power at 60 Hz on the power distribution line. A bilaterally conducting thyristor would produce 120 pulses per second. Each cycle starts at zero ballast output voltage. As noted above, the starting pulse is required by ANSI standards to be present when the ballast output voltage is at or near its peak value.
When the starter aid just described initiates ionization of the gas in the lamp, ballast current begins to flow and the ballast output voltage drops and remains low during normal lamp operation. This normal operating ballast output voltage is not high enough to allow capacitor 14 to charge to the breakover voltage of diac 20. Therefore, the starting aid only supplies starting pulses during starting of the lamp, but not thereafter. The operation is automatic.
Although the circuit just described is useable and meets the ANSI specification for many lower-wattage ballasts, for lamp wattage of higher values, the specifications are increasingly harder to meet with the circuit described in FIG. 2. That is, it is extremely difficult to maintain the pulse position, width and amplitude at such higher wattage conditions. This is especially true, as all parameters must be met for variations in line voltage as specified by ANSI. Further, there are additional weaknesses in the circuit as a result of the presence of power resistor 12.
For example, the charge times of capacitor 10 and of capacitor 14 are dependent on and extremely sensitive to the amplitude of the voltage on the power distribution line. As this voltage amplitude varies, so does the pulse position, pulse width and the amplitude of the voltage on capacitor 10, and hence the time that SCR 22 conducts and discharges the capacitor.
A second problem with resistor 12 is that it consumes power and generates heat. This is a problem at any time, but especially under no-lamp or end-of-lamp-life conditions.
Another problem with the FIG. 2 circuit is the reliability of many of the components, especially of SCR 22 and capacitor 10, since operation in the above manner stresses each of these components greatly.
Therefore, it is a feature of the present invention to provide an improved lamp starting aid for providing pulses to a high intensity, gaseous discharge lamp which aid does not include a power resistor.
It is another feature of the present invention to provide an improved lamp starting aid for providing pulses to a high intensity, gaseous discharge lamp which reduces the number of components related to establishing pulse position, width and amplitude when compared with prior art circuits and, therefore, makes it easier to meet the ANSI specifications for such circuits at high wattage operating conditions.